(数字化合1)普克尔效应.doc

实验一 普克尔效应 实验目的 验证偏振光中的普克尔效应 测量普克尔单元的半波电压 实验概述 当自然光射到一些特殊的晶体表面时，会被分解成两束光，一束叫寻常光（o光），符合普通的折射定律，另一束则不符合，叫非常光（e光）。其折射率no和ne合称为晶体的主折射率。LiNbO3在强电场作用下能够变成双折射物质，由于o光、e光在晶体内的速度不同，通过晶体后，两光间产生相位差。进而在屏幕上出现干涉图样。改变电压值，干涉图样中的双曲线系会通过中心移向另外的线系。 实验原理 The Pockels effect is the name given to the occurrence of birefringence and to the change in existing birefringence phenomena in an electric field linearly proportional to the electric field strength. It is related to the Kerr effect, although in the latter case the birefringence increases exponentially with the electric field strength. For reasons of symmetry, the Pockels effect can only occur in crystals with no inversion center, whereas the Kerr effect can occur in all substances. When the direction of the light beam and the optical axis of birefringence are perpendicular to each other, we call this a “transverse configuration” (see Fig. 1). The electric field is applied in the direction of the optical axis. For Pockels cells in the transverse configuration, lithium niobate (LiNbO3) is most often used. Lithium niobate crystals are optically uniaxial, negatively birefringent and have the main refractive indexes no = 2.29 for the ordinary beam, and ne = 2.20 for the extraordinary beam (measured using the wavelength of the He-Ne laser, l =632.8 nm. Birefringence in a conoscopic beam path The proof of birefringence in a conoscopic beam path is described in numerous optics textbooks. A crystal with planeparallel cut faces is illuminated with a divergent, linearly polarized light beam, and the light passing through it is observed behind a perpendicularly aligned analyzer . The optical axis of the birefringence is clearly apparent in the interference image, as it is indicated by the symmetry in its vicinity. In this experiment, the optical axis is parallel to the entrance and exit surfaces; this is why the interference pattern consists of two sets of hyperbolas which are rotated by 90 with respect to one another. The real axis of the first hyperbola set is parallel to the optical axis, while that of the second set is perpendicular to the optical axis. The dark lines of the interference image are caused by light rays for which the difference between the optical paths of the extraordinary and the ordinary partial beam in the crystal is an integral multiple of the wavelength. These light rays retain their original linear polarization after passage through the crystal, and are extinguished in the analyzer. The light rays reaching the center of the interference image are normally incident on the surface of the crystal. For these rays, the path difference between the extraordinary and the ordinary partial beam is = d × (no - ne), (I) where d = 20 is the thickness of the crystal in the direction of the beam. The path difference corresponds to approximately 2800 wavelengths of the laser light used. however, is not usually precisely a whole multiple of , but rather lies between two values, m = m and m+1 = (m + 1) . The dark lines in the first hyperbola set thus correspond to the path differences m+1, m+2, m+3, etc., and those of the second set to m, m-1, m-2, etc. (Fig. 3). The position of the dark lines, or better their distance from the center, depends on the magnitude of the difference between and m. Fig. 3: Interference pattern in the conoscopic beam path with the optical axis of the crystal in the direction of the arrow. The numbers represent the path difference between the ordinary and the extraordinary partial beam. Thus for example the lines with the value +1(-1) have the path difference m+1 (m-1) The Pockels effect magnifies or reduces the difference of the main refractive indices no – ne, depending on the sign of the applied voltage. This in turn alters the difference – m × l, and thus the position of the dark interference lines. If the so-called half-wave voltage Up is applied, the value of is changed by one-half wavelength. The dark interference lines shift to the positions of the bright lines, and vice versa. This process repeats itself each time the voltage is increased by Up. 安全事项 不要直视出射或反射的激光。 高压危险 实验装置及调试 元件的具体布置参照图4。 光学元件的布置 --依次安装He-Ne激光器、5-mm透镜(a)、50-mm透镜(b)和半透明屏，仔细调整好激光器和5-mm透镜的高度，使得50-mm透镜得到最适宜的照明。在屏上得到一个均匀明亮的光场。 --安置检偏器，转动其黄色手柄来改变检偏器的偏振方向，直到屏上的亮度最暗。 --将普克尔盒放置到元件队列中，务必保证两个高压插头已插接到位。将其滑动到一个精确的位置，要求该处的激光束的横截面最小。 转动其手柄，定在相对检偏器旋转+45°or -45°的位置。 细调 --调整激光、5-mm透镜和普克尔单元的高度直到干涉图样的双曲线系的中心位于视野的中心。 --如果必要，沿支架杆轴线转动普克尔单元。 电连接 高压危险。请始终保持电线与电源、普克尔单元的良好连接。 --将普克尔单元与高压电源的左输出相连（最大短路电流100μA），确保负端与地相连。 --将电压源的电压计始终旋到左端，打开高压源的开关，用选择钮激活左路输出。高压U不超过2 kV --要避免对普克尔晶体的较大的电冲击，故关闭电源和变换极性时，一定要先将电压降至0v后再操作。 实验步骤 a)验证双折射 --观测开始时干涉图中双曲线系的位置和普克尔单元基准点的位置。 --转动手柄，慢慢改变普克尔单元基点位置，描述干涉图样的改变，并回答 （1）第一双曲线系的实轴是否总平行于晶体的光轴（以基点的方向标示）？ （2）屏幕上干涉图样最清晰和最模糊时，光轴和检偏器的夹角值分别为多少，为什么？， b)验证普克尔效应 --将普克尔单元的基点旋回至起始位置（相对检偏器±45°） --缓慢升高电压U（不超过2 kV），观测干涉图的改变。 --将电压降至0 V，将高压电源的正负插头对调，即普克尔盒的电连接反向。 --再次升高电压U（不超过2 kV），观测干涉图的改变。 （3）随着电压的改变，一个线系的曲线通过中心移向另外的线系，为什么？ c)测定半波电压 --电压设为0v，用一粗笔在纸上描出干涉图样的暗条纹。 --缓慢升高电压，记录下干涉的明条纹恰好移到纸上的暗条纹位置时的电压改变值（即半波电压值），并求出平均值。 思考题 1， 使用现有的光学元件检测激光器所发出的光的偏振特性。 2， 透镜1和透镜分别为什么透镜，他们在光路中起到什么作用，满足什么条件时，才能完成实验，单独使用，是否能再现实验现象。要求应用光学的相关知识，配以光路图进行解释。 3， 简述锥光干涉的相关知识。 4