以旋光外差干涉術測量雙折射晶體之尋常光及非尋常光折射率

Abstract

雙折射晶體常用於許多的光學元件，諸如雙折射稜鏡、波片等。本研究提出利用旋光外差干涉術來測量雙折射晶體之尋常光及非尋常光折射率的方法。首先，一線性偏振光通過電光晶體調變器與四分之一波片後，可得一具有頻差之旋光外差光源。其次，當光線經過雙折射晶體反射並通過檢偏板後，根據其反射時之物理特性及Jones calculus，可得測試光強度為一弦波信號。由於參考光及測試光間的相位差是晶體方位角及入射角的函數，將測量所得之相位差值代入此函數中，即可同時求出待測物之尋常光及非尋光折射率。由於本方法利用光束反射時所產生的相位差來推導，因此與待測晶體的厚度無關且待測物表面也不需要有良好的平整度。此外，本方法是使用共光程外差干涉儀，因此系統之穩定度及解析度均高。

Birefringent crystals are often used to fabricate many optical devices, such as birefringent prisms and wave plates. In this thesis, a method for measuring the ordinary and the extraordinary refractive indices of a birefringent crystal by using circularly polarized heterodyne interferometry is presented. Firstly, a linearly polarized laser with an electro-optic modulator and a quarter-wave plate is used to act as a circularly polarized heterodyne light source. Next, according to Jones calculus and the reflective characteristics of the birefringent crystal, the test signal intensity with a cosine wave form can be obtained after the light reflected from the crystal passes through an analyzer. The phase differences between the reference beam and the test beam are the function of the azimuth angle of the crystal and the incident angle, and the ordinary and the extraordinary refractive indices can be estimated simultaneously by substituting the measured values of phase differences into the above function. Because only the reflective characteristics of a light beam are used, the thickness and the surface flatness of the test material are not required. In addition, it is very stable and has high resolutions owing to its common-path heterodyne interferometric structure.