To achieve data-driven intelligence in engineering applications, the key requirements for distributed optical fiber sensor networks are large capacity, long distance, dense distribution, fast response, and high signal-to-noise ratio (SNR). Combining the traditional optical reflectometry techniques with the large-scale fiber Bragg grating (FBG) array fabricated on-line by a fiber drawing tower, multiplexing and demodulation of thousands of FBG sensors is successfully demonstrated, which greatly expands the application of FBG sensors to the field of distributed sensing. In this work, the fabrication, demodulation, and applications of large-scale FBG arrays are reviewed. Firstly, the on-line fabrication technology and process of large-scale FBG arrays are introduced. Secondly, the demodulation techniques including wavelength demodulation based on optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR), as well as phase demodulation based on phase-sensitive OTDR are reviewed, where we mainly focus on the key parameters such as multiplexing capacity, demodulation speed, spatial resolution, and SNR. Last, the applications of the large-scale FBG arrays are briefly discussed.

We developed a large multiplexing capacity dense ultra-short (DUS)-FBG array for high spatial resolution distributed sensing applications. With identical central wavelength and low peak reflectivity (−40 dB), all the ultra-short FBGs share the short length (1 mm) and extremely small spacing (500 μm). The large multiplexing capacity and lower crosstalk of the DUS-FBG is investigated through both simulation and experiment. Use of DUS-FBG array interrogated by optical frequency domain reflectometry (OFDR) for distributed temperature and non-uniform strain sensing was conducted. We demonstrated high spatial resolution over 6680 FBGs along a 10 m-long fiber, and temperature and strain precision of 1.00 °Cand 20.02 με. Distributed temperature and non-uniform strain sensing experiment results are consistent with the theoretical analysis, and verify the high spatial resolution and large multiplexing capacity.

Distributed static and dynamic sensing is demonstrated with an ultra-short fiber Bragg grating (USFBG) array. The USFBGs serve as the sensors and reflection mirrors at the same time. Distributed static sensing is performed by demodulating the strain-induced or temperature-induced wavelength shift of each USFBG. Dynamic vibration sensing is realized based on phase variation between two adjacent USFBG reflected pulses. Static temperature and dynamic vibration are applied to the sensing ultra-short FBG array simultaneously. The experiments show that the temperature measurement from 30 °C to 80 °C is achieved successfully. And dynamic sensing of nε scale vibration and 12.5 kHz acoustic wave are demonstrated at a sampling rate of 50 kHz.

Tracking operating trains and identifying illegal intruders are two important and critical issues in subway safety management. One challenge is to find a reliable methodology that would enable these two needs to be addressed with high sensitivity and spatial resolution over a long-distance range. This paper proposes a novel monitoring approach based on distributed vibration, which is suitable for both train tracking and incursion detection. For an actual subway system, ultra-weak fiber Bragg grating (FBG) sensing technology was applied to collect the distributed vibration responses from moving trains and intruders. The monitoring data from the subway operation stage were directly utilized to evaluate the feasibility of the proposed method for tracking trains. Moreover, a field simulation experiment was performed to validate the possibility of detecting human intrusion. The results showed that the diagonal signal pattern in the distributed vibration response can be used to reveal the location and speed of the moving loads (e.g., train and intruders). Other train parameters, such as length and the number of compartments, can also be obtained from the vibration responses through cross-correlation and envelope processing. Experimental results in the time and frequency domains within the selected intrusion range indicated that the proposed method can distinguish designed intrusion cases in terms of strength and mode.

It is challenging for engineers to timely identify illegal ground intrusions in underground systems such as subways. In order to prevent the catastrophic collapse of subway tunnels from intrusion events, this paper investigated the capability of detecting the ground intrusion of underground structures based on dynamic measurement of distributed fiber optic sensing. For an actual subway tunnel monitored by the ultra-weak fiber optic Bragg grating (FBG) sensing fiber with a spatial resolution of five meters, a simulated experiment of the ground intrusion along the selected path was designed and implemented, in which a hydraulic excavator was chosen to exert intrusion perturbations with different strengths and modes at five selected intrusion sites. For each intrusion place, the distributed vibration responses of sensing fibers mounted on the tunnel wall and the track bed were detected to identify the occurrence and characteristics of the intrusion event simulated by the discrete and continuous pulses of the excavator under two loading postures. By checking the on-site records of critical moments in the intrusion process, the proposed detection approach based on distributed structural vibration responses for the ground intrusion can detect the occurrence of intrusion events, locate the intrusion ground area, and distinguish intrusion strength and typical perturbation modes.

In order to monitoring vibration-reduction and vibration-isolation effects of subway floating slab, an FBG displacement sensor was developed to monitor the deformation of the subway floating slab. The proposed sensor mainly consists of an FBG, a thin-walled ring, a steel-spring, connecting rod probe, linear bearing, sleeve, and other auxiliary unit. The connecting rod probe transmits the external displacement to the steel-spring and elastic thin-walled ring, and then leads to the wavelength shift of the FBG pasted in the reserved groove of the thin-walled ring. Meanwhile, the linear bearing can reduce the friction loss and increase the sensor stability. Lots of sensing tests showed that, the sensor has a sensitivity of 36.36 pm/mm and a precision of 8.25×10-2 mm in the displacement range of 0 - 20 mm, and also has good micro-displacement measurement capabilities. The theoretically calculated data for the vertical displacement of the floating slab are consistent with the actual measured data, which verifies the feasibility of the proposed displacement sensor in the monitoring of the subway floating slab..