Common-Mode Noise Suppression Technique in Interferometric Fiber-Optic Sensors

Efficient suppression of common-mode noises (CMNs) is significant for the performance of interferometric fiber-optic sensor system. To suppress CMNs, a reference interferometer is normally required as noise canceller. In this paper, we firstly report a detailed correlation analysis between the outputs of the sensing and reference interferometers. By using a band-limited noise decomposing method, we show that the correlation coefficient of the two interferometer outputs is a strong function over their mean phase difference, which appears as the limiting factor of the CMN suppression effect. Based on the analysis, we propose a novel CMN suppression technique by locking the mean phase value of the sensing interferometer using a reference interferometer with a 3 × 2 coupler. Experimental results show that our technique can increase the correlation coefficient between the outputs of sensing and reference interferometers by 6% (to 0.93) and reduce its standard deviation by $\sim$70% compared to traditional method. The CMNs suppression factor is measured as $-$8.49 dB in the frequency range from 300 Hz to 1 kHz, comparing to $\sim -$6 dB achieved by traditional method of simple subtraction. Our results set a new benchmark on the noise level of interferometric fiber-optic sensors.