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A difference interferometric refractive-index (RI) sensor is based on the differential phase modulation between the two orthogonal polarization (TE and TM) modes of guided light beams of optical waveguide. [1][2][3][4] We have developed such a type of RI sensor using a slab optical waveguide (SOWG) with a prism coupler. 5This type of RI sensor allows us to use a relatively simple optical configuration, compared with other RI sensors using a planer optical waveguide, such as a Mach-Zehnder interferometer-type sensor. 6,7 However, there are some difficulties concerning the detection system. Although we applied an absorbance detection system to the RI sensor in previous work, 5 its sensitivity was dependent upon the D0 value in Eq. (2) in the Theory section; the D0 value is involved in the coefficient in the expression for the relation between the absorbance and the RI change of the sample solution. The D0 value could change in each experimental setup, and optimization of the measurement conditions was a tedious task. 5 On the other hand, other researchers have used rather complex detection systems to avoid such a problem; multiple photo-detectors systems or a charge-coupled device (CCD) camera system were applied when complicated signal processing was required. [1][2][3][4] In this paper, we describe a simple detection system for the RI sensor using lock-in modulation. This system enables us to easily obtain the optimum measurement conditions. Moreover, its experimental setup is much simpler than those of other works;1-4 a single-beam configuration can be applied and complicated signal processing is not necessary. Theory