We propose a method for small displacement measurement based on the angle deviation to phase change transformation. The phase change of common-path heterodyne interferometry due to the angle deviation of incidence of a light at interfaces caused by the displacement is detected by a lock-in amplifier. To obtain more accurate results we used an angular amplifier to increase the angle deviation and utilized a surface plasmon resonance (SPR) sensor to enhance the performance of phase detection. When a translator moves one of two face-to-face plane mirrors at an end and then rotates it a small angle, a light is incident onto the mirrors and reflected N times. The outgoing light is also deflected N times of the angle and incident into a SPR sensor. Thus the phase shift due to the angle deviation is amplified N times. The accumulated phase shift is proportional to the amplified angle deviation and displacement. Therefore, the phase change is obtained and the displacement is measured. The amount of movement required can be as low as 0.13 μm without an SPR sensor or 0.08 μm with an SPR sensor. The maximum measurement range can reach 1000 μm.
A new measuring method based on the common-path heterodyne interferometry is used to determine the twist angle, pre-tilt angle, and cell gap of an optically compensation bend (OCB) liquid crystal (LC) cell. In this paper, the twist angle of an OCB LC cell can be determined by measurement the phase difference instead of the reflectance difference. Because the measuring system is based on the common-path heterodyne interferometry, it can provide the most sensitive way to measure the twist angle of an OCB LC cell against the air turbulence and environmental vibrations. The method has some merits, e.g., a simple setup, in real-time test and reliability, etc.
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