Simultaneous Differential Interrogation for Multiple FBGs Based on Crossed Sagnac Loops and Microwave Network

Conventional intensity-based fiber Bragg grating (FBG) demodulation systems are usually vulnerable to the power fluctuations and not convenient to locate the positions of the gratings, which set drawbacks to their large scale sensing applications. Here, we present and demonstrate a simultaneous differential interrogation method for multiple FBGs based on crossed Sagnac loops and microwave networks for the first time. With the help of the unique characteristics of the Sagnac filters, Gaussian-shape nature and numbers of passband channels, a linear behavior can be achieved. What's more, the utilization of the microwave network can not only contribute to the positioning of the gratings, but also increase the response speed and cut down the cost for the mature microwave technology. The experimental results exhibit high linearity and good immunity to the interference between adjacent gratings and the variations of power. A strain sensitivity of ~0.0034 dB/με in the range from 0 to 2000 με and a spatial resolution for FBG locating of 0.1 m have been obtained. The changes of the impulse peak ratios are less than 0.02 dB when 2 dB power reduction is introduced. Furthermore, numerical studies about the influences of several important parameters to the capacity and performance of the sensing system have also been carried out, indicating that it has the ability of multiplexing thousands of FBGs covering at least several kilometers.