本研究提出一套創新的「外差式共光程折射率量測技術」，用以量測透明固體及液體折射率及其折射率變化。此套量測技術採「共光程光路」為設計概念，使參考光與量測光於空間中的行進路徑幾乎相同，可有效降低環境對量測結果所造成的影響，同時透過外差干涉術的使用，使此套「外差式共光程折射率量測技術」具備高穩定性及高解析度。
此套「外差式共光程折射率量測技術」由外差光源、擴束系統、特製半圓形半波片、透明壓克力盒、旋轉平台、全像光柵和相位解調系統所組成。藉由電光調制技術來產生外差光源，而後使外差光源經共光程光路後形成參考光及量測光，其中量測光將穿過放置於壓克力盒內之待測物，利用聚焦透鏡將量測光及參考光聚焦於光柵上，使繞射之量測光及參考光部分區域相疊合後形成干涉條紋。當待測物之折射率改變時，量測光所行經之光程將隨之發生變化，藉由計算干涉訊號之相位變化量，即可回推待測物之折射率變化量。此外，當待測物於空間中產生旋轉角變化時，亦會改變量測光所行經之路徑，使得干涉訊號產生相位變化，透過計算其相位變化量，即可回推待測物之絕對折射率。
為了驗證此套「外差式共光程折射率量測技術」的可行性及其性能，我們依序進行絕對折射率、折射率變化及濃度變化等量測實驗，並將量測結果與參考文獻進行比對。由實驗結果證明，此套「外差式共光程折射率量測技術」具備高解析度(3.4×10-7 RIU)、高靈敏度(2.6×106 /RIU)，及高穩定度(1.3×10-7 RIU/min)的量測能力，可廣泛應用於需高精密折射率量測的場合中。

An innovative heterodyne common-optical-path (COP) refractive index (RI) measurement technique for measuring RI of transparent or semi-transparent solid or liquid sample is proposed in this study. The design concept of the proposed measurement technique is based on the COP configuration. According to the measurement principle of COP configuration, the effects resulting from experimental disturbances can be effectively decreased due to the moving paths of the reference and the measurement beams are almost the same. Moreover, by using the technique of heterodyne interferometry, the proposed heterodyne COPRI measurement technique has the ability of high stability and high resolution.
The proposed heterodyne COPRI measurement technique is composed of a heterodyne light source, beam-expander, specific semi-circle half-wave-plate, rotation stage, holographic grating, and phase demodulation system. A heterodyne light source is generated by using the electro-optics modulating technique. The reference and measurement beams are formed as the heterodyne passes through the COP configuration. The beam passes through the sample which placed in a PMMA box is regarded as measurement beam. By using a suitable focusing lens, the reference and measurement beams are focused on the grating and then diffracted. Interference patterns can be formed since the diffracted beams of reference and measurement beams are overlapped partially. The optical path of measurement beam will vary as the RI of sample changes. Therefore, the value of RI variation can be obtained by calculating the phase variation of the interference signal. Furthermore, the optical path of measurement beam will be changed as the sample is rotated by a rotation stage. When this situation occurs the phase of interference signal will be changed. In this case, the RI value of sample can be acquired by calculating the phase variation of the interference signal.
In order to demonstrate the facility and performance of our proposed heterodyne COPRI measurement technique, the experiments of absolute RI, RI variation and consistency variation are successively performed and compared with the values revealed in references. As can be proved from the experimental results, the proposed heterodyne COPRI measurement technique, has the ability of high resolution (3.4×10-7 RIU), high sensitivity (2.6×106 /RIU), and high stability (1.3×10-7 RIU/min), can be widely applied in the fields that required high precise refraction index measurement.

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