## 1 Introduction  
Optical birefringence causes two orthogonally polarized propagating lights to have different refractive indices in a birefringent medium. The birefringent materials have been widely used in optical sensors [1; 2; 3; 4; 5; 6; 7; 8; 9], polarization holographs [10], waveplates [11] and polarization controllers [12]. Usually, the uniaxial or biaxial crystals with anisotropic properties have polarization-dependent refractive indices. It is possible to make a phase delay between two orthogonal polarizations. It is well known that the reflected light can induce the phase delay of orthogonal polarizations through the principles of surface plasmon resonance (SPR) and total internal reflection (TIR) [13; 14; 15; 16]. The variation of phase delay can be measured by utilizing a pair of polarizers according to polarization interferometry. The transmitted birefringent refraction (BR) also can cause the phase delay. Anisotropic materials such as bulk and liquid crystals have been proposed and evaluated for application in optical sensors. In various solid birefringent crystals, the photoelastic, thermo-optic and electro-optic characteristics were adopted for the pressure, temperature and electric field transducers, respectively [1; 2; 3; 4; 5; 6]. The birefringent fiber or planar waveguides are also flexible for the compact designs in integrated optic sensors [7; 8]. Recently, the liquid crystals have been successfully demonstrated in the application of biosensors due to the birefringence changes induced by the bioreaction [9]. It is essential to monitor liquid concentrations during manufacture and transportation [17; 18]. Especially, some chemical solutions are corrosive and a specific metallic sensing head is necessary to monitor the concentrations based on the conductivity dependence response in electronic signals [19]. Since optical probe metrology has remote and non-contact advantages, it can provide a flexible measurement setup. In normal situations, the refractive index (RI) of liquid is concentration-dependent when mixed with different contents. The RI variations of the tested liquids can be detected according to the SPR and TIR sensors in the polarization interferometer [13; 16]. Therein, the phase interrogation and spectra interrogation are less disturbed by the intensity noise of the probe lights. Typically, the optical SPR sensor is fabricated by coating a thin metallic gold film onto a prism or glass sheet. To achieve the highest measurement sensitivity, the thickness and complex RI of the gold film, the incident angle and the probe wavelength have to meet the SPR requirements. Moreover, the tight resonance conditions limit the dynamic range. Although the SPR technique exhibits more sensitivity in comparison with the TIR one, the intrinsic characteristics of the coated metallic layers are easily destroyed in harsh chemical solutions. In a commercial optical instrument for concentration measurements of chemical solutions, the TIR principle utilizing a chemically resistant prism is a typical design for monitoring dynamic concentrations. Since the detected resolution is limited by a beam spot size and a spatial resolution in the CCD camera, it is difficult to improve the concentration resolution down to 0.05 wt. % [20]. The KTP crystal is an important material used for nonlinear optics because of its high resistance to optical damage at short wavelengths. Most of the applications are demonstrated on the wavelength converters [21]. To realize efficient wavelength conversion, a phase-matching condition is achievable under the birefringence effects. For example, it is well known that the green light of 532 nm can be generated by utilizing secondary harmonic generation pumped by a wavelength of 1064 nm. A periodic poling KTP is adopted for various wavelength generations and optical quantum communications [22]. In the previous report [23], the KTP plate has been studied for applications on the concentration measurements of salt liquid by utilizing the BR principle.