STUST

1. Transmitted Birefringent Sensor for Optical Displacement Measurement Ching-Shing Wang, Jhao-Heng Wang, and Ruey-Ching Twu* Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology Tainan, Taiwan *E-mail: rctwu@mail.stust.edu.tw STUST Abstract A novel transmitted-type birefringent sensor is proposed to measure displacement in a heterodyne interferometer. The sensitivity of phase versus displacement can be improved by using an immersed KTP plate in a glycerin solution. Introduction The birefringent materials can make a phase delay between two orthogonal polarizations of a propagating light which are widely used for waveplates and electro-optic modulators [1,2]. Usually, the phase delay is dependent on a birefringence and thickness of a birefringent plate (BP), and an incident angle of the incident light [1]. Moreover, the relative refractive index between the birefringent plate and the environment also will change the phase delay values. When the incident light has a lateral displacement onto a spherical lens, the displacement will make a refractive angle change. The lens is a useful transducer to transfer the displacement to the angle changes. In the past decades, a prism-type surface plasmon resonance (SPR) sensor has been widely used in an angular displacement measurement [3]. By combing the lens and the SPR devices, the optical displacement measurement apparatus has been demonstrated successfully [4,5]. Although the SPR-based displacement measurements can achieve a high resolution, a limited distance range and a specific incident angle are the main drawbacks. In this paper, a novel optical displacement transducer has been evaluated by using a glass cylinder embedded with the BP immersed in a glycerin solution. The proposed BP transducer has a compact size in comparison with the SPR-based one. By the way, this transducer has more flexible for setting the initial incident angle. The dynamic range of measurement is better than the SPR-based one. Experimental Setup (b) (a) Fig. 1 (a) Schematic diagram of an optical displacement measurement apparatus and (b) transducer. Experimental Setup 40μm 40μm Fig. 2 Phase variation as a function of refracted angle in the displacement sensor. Fig. 3 Phase variation as a function of sinusoidal displacement shifts: (a) LN plate and (b) KTP plate. Conclusion In conclusion, a simple and novel transmitted-type birefringent sensor was successfully demonstrated to measure a small displacement in a heterodyne interferometer. The displacement measurement sensitivity can be improved by using an immersed KTP plate in a glycerin solution. The compact transducers have advantages of less precise angle requirements of the incident light and wider displacement range than conventional SPR-based transducer. Reference [1] C. H. Hsieh, C. C. Tsai, H. C. Wei, L. P. Yu, J. S. Wu, and C. Chou, Appl Opt, 46 (2007) 5944-5950. [2] K. H. Chen, W. Y. Chang, and J. H. Chen, Opt Express, 17 (2009) 14143-14149. [3] H. P. Chiang, J. L. Lin, R. Chang, and S. Y. Su, Opt Lett, 30 (2005) 2727-2729. [4] J. Y. Lin, K. H. Chen, and J. H. Chen, Opt Laser Eng, 49 (2011) 811-815. [5] S. F. Wang, Sensors, 9 (2009) 2498-2510. [6]R. C. Twu, Y. H. Lee, and H. Y. Hou, Sensors 10 (2010) 9609-9619.