STUST

1. Sensitivity Enhanced Optical Angle Measurement Using an Immersion Type Birefringent Sensor Ruey-ching Twu*, Ching-Shing Wang, and Jhao-Sheng Wang Department of Electro-Optical Engineering, Southern Taiwan University of Science Technology, Tainan, TAIWAN E-mail: rctwu@mail.stust.edu.tw STUST Abstract . In this paper, a novel immersion type birefringent sensor is proposed for a optical angle measurement in a heterodyne interferometry. The sensitivity of optical phase change versus to incident angle is about 630 by utilizing a KTP plate immersed in a glycerin solution. Introduction The angular displacement measurement is essential in a various mechanical apparatus. For example, a high precision exposure machine needs fine angle control for the reticle alignment to ensure well photolithographic fabrication process. Optical remote measurements have advantages of non-contact and easily performed with an optical interferometer. Among of the various reflection measurement schemes, surface plasmon resonance (SPR)-based methodology and total internal reflection (TIR) are widely used for optical angle variation measurements [1-3]. In the SPR approach, a resonance between the incident light and surface plasmon wave is dependent on the incident angle. The reflected intensity is smallest and phase change is abrupt under the resonance condition. Therefore, the phase interrogation can provide a higher measurement precision, and also better than the TIR method. However, the highest sensitivity limits the dynamical measurement range. The thickness control of thin metallic films (Au or Ag) onto the SPR prism, and the initial angle are also critical for achieving enough measurement resolutions [4]. We present a new technique for the optical angle measurement using an immersion birefringent (BR) sensor performed in a common-path heterodyne interferometer. In the technique, the BR plats (LN and KTP) were immersed in different medium such as air, water, and glycerin. The optical phase delay between two orthogonal polarizations is dependent on an incident angle, a thickness of BR plate, and refractive index of immersed solution. In comparison with the SPR and TIR schemes, the technique does not require the precisely initial angle setting. It also can provide enough sensitivity and wide dynamic range detection. Measurement Setup Fig. 1. Measurement setup and OAS structure. Experimental Setup Fig. 3. Phase change versus angle for MgO:LN and KTP in a water medium. Fig. 4. Phase change versus angle for MgO:LN and KTP in a glycerin medium. Fig. 2. Phase change versus angle for MgO:LN and KTP in an air medium . Conclusion A simple immersion type birefringent sensor was proposed and evaluated for the applications of optical angle measurement. The results show that the system measurement resolution can reach to 2.85×10-5 deg by using the transducer of KPT plate immersed in a glycerin medium. Reference [1] H. P. Chiang, J. L. Lin, R. Chang, and S. Y. Su, “High-resolution angular measurement using surface-plasmon-resonance via phase interrogation at optimal incident wavelengths,” Opt. Lett.30, 2727-2729 (2005). [2] J. Guo, Y. Gao, Z. Zhu, and W. Deng, “Small-angle measurement based on surface-plasmon resonance and the use of magneto-optical modulation”, Appl. Opt. 38, 6550-6555 (1999). [3] M. H. Chiu, S. F. Wang, and R. S. Chang, “Instrument for measuring small angles by use of multiple total internal reflections in heterodyne interferometry”, Appl. Opt. 43, 5438-5442 (2004). [4] Y. H. Huang, H. P. Ho, S. Y. Wu, S. K. Kong, W. W. Wong, and P. Shum, “Phase sensitive SPR sensor for wide dynamic range detection,” Opt. Lett. 36, 4092-4094 (2011). [5] R. C. Twu, Y. H. Lee, and H. Y. Hou, “A comparison between two heterodyne light sources using different electro-optic modulators foroptical temperature measurements at visible wavelengths,” Sensors10, 9609-9619 (2010). [6] K. Kato and E. Takaoka, “Sellmeier and thermo-optic dispersion formulas for KTP,” Appl. Opt. 41, 5040-5044 (2002).