Demonstration of multi-channel fiber optic interrogator based on time-division locking technique in subway intrusion detection.

Low noise, good consistency, and long-term stability are critical for a multi-channel fiber optic intrusion detection system. This paper proposes a low-noise 32-channel fiber optic interrogator based on phase generation carrier technique, which emphasizes on the analysis of key parameters related to system consistency and stability. A novel time-division locking technique of the carrier modulation depth and carrier phase delay is proposed. By locking the carrier modulation depth and carrier phase delay simultaneously, the consistency and stability of the multi-channel interferometric interrogator is significantly improved. The results show that in the multi-channel system, the noise level of each channel is below 12.6 ng/√Hz @ 5 Hz, and the noise level differences between each two channels are less than 2.6 dB, the noise level fluctuations of all channels are less than 1.4 dB over 24 hours. A subway intrusion detection is demonstrated in Shenzhen, China. Geological drilling events can be clearly recorded and identified.