| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 103 === 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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