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A phase-noise suppression technique is presented for the fibre-optic interferometric sensor (FOIS). The scheme is based on the use of an optical reference signal synchronically acquired with the sensing signal and the heterodyne demodulation technique. The reference signal has the same phase noise originating from the laser and the acousto-optic modulator (AOM) driver with the sensing signal. The phase noise of the FOIS is suppressed in the phase demodulation process by using the synchronous reference signal without additional signal processing. This method can also reduce the requirements on the noise performance of the AOM driver for the heterodyne demodulation technique, thus it can be more widely used in practical applications. Experimental results show that the frequency noise induced phase noise can be reduced effectively and a residual phase noise floor of lower than -100dB has been achieved over a frequency of higher than 300Hz.