MIOC-S chip is a key component in Fiber Optic Gyroscope (FOG) for measuring angular velocity of an inertial navigation system. This integrated device, fabricated on Lithium Niobate (LiNbO3) material, is composed of a polarizer, a Y-junction coupler and dual electro-optical phase modulators. Waveguides on the chip are produced by the High Temperature Proton Exchange (HTPE) process. The MIOC-S has more than 60dB Polarization Extinction Ratio (PER) and can minimize bias drift resulting from polarization crosstalk induced non-reciprocity. This MIOC-S device has been used in a high-sensitivity inertial navigation system to detect less than 0.5 deg/hour angular velocity change. It has highly reliable performance with wide temperature range tolerance.