Automatic bias control of electro-optic modulator in optical-fiber Brillouin sensing system

Electro-optic modulator (EOM) is widely used in optical fiber Brillouin sensing system. However, the characteristics of EOM are sensitive to the external temperature change and mechanical vibration. It manifests a time-dependent drift of transmission curve, which leads to the decrease of extinction ratio and measurement accuracy of the sensing system. In order to set a stable EOM operation point for pulse modulation of input lightwave, an automatic bias control system with optical feedback is designed in this paper based on the observation of drift phenomenon of transmission curve and the measurement of half-wave voltage of EOM. This system can control the bias more precisely and steadily than the previous methods. Experimental results indicate that the extinction ratio of output pulse can be as high as more than 28 dB and keep stable in the whole measurement time of three hours with this system. The steady bias control and obtained high extinction ratio provide a foundation of high accuracy for optical-fiber Brillouin sensing system.