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A self-homodyne optical coherent detection system is demonstrated using polysilicon surface micromachining technology. Fabricated on a Silicon chip, such a system comprises a polysilicon membrane as an optical phase modulator, a micro-beamsplitter and two micro-mirrors for beam steering. A signal Max/Min ratio of 22 dB is achieved with this coherent detection system. Various types of sensors could be potentially made out of this system, such as an optical index sensor with a minimal detectable index change down to 10/sup -7/, chemical and gas sensing, and laser spectrometry. The optical coherent sensing method is also applicable to the signal readout of inertial sensors, and an example of a vibration sensor is presented.