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.

An integrated fiber Michelson interferometer (MI) based on an asymmetrical twin-core fiber (TCF) cascaded with a side-hole fiber (SHF) for multi-parameter sensing is proposed and demonstrated. The asymmetrical TCF consists of a centric core and an eccentric core with same refractive index. The centric core of the TCF is aligned with the core of the SHF and the eccentric core is aligned with one air hole of the SHF. Because the eccentric core of the TCF is off the axis of the fiber, a bend or twist will cause a variation of the optical path difference (OPD) of the MI, which makes the MI structure be suitable for twist and vector curvature measurements. Experimental results show that the bending sensitivities of the sensor are -6.968 nm/m-1 and 6.978 nm/m-1 at the bending directions of 0° and 180° in the curvature ranges of 0 m-1 to 10.708 m-1, respectively, and the twist sensitivity of the sensor is 0.639 nm/(rad/m) in the twist ranges of 0 rad/m to 7.44 rad/m. The structure is also sensitive to strain and temperature, and the strain and temperature sensitivities of the interferometer are 1.36 pm/ $\mu\epsilon$ and 10.37 pm/°C, respectively.

We combine wavelength-division multiplexing (WDM) and frequency shifted interferometry (FSI) to interrogate a large-scale ultra-weak fiber Bragg grating (FBG) array. Based on Sagnac interference, FSI can location resolve sensors without using pulses or fast detection, therefore considerably lowers the system cost. By combining FSI with WDM, higher spatial resolution can be achieved. We demonstrated simultaneous interrogation of 363 FBG sensors, grouped into 121 identical units of 3 FBGs of different central wavelengths and spaced two meters apart. Based on the performance of the 363 grating system, we show the potential for interrogating 3207 sensors with good signal-to-noise ratio. Stability test and temperature sensing were carried out, and the obtained temperature resolution was ± 0.4 ° C. The results indicate the proposed scheme can greatly enhance the multiplexing capacity and meet the requirements of large-scale optical fiber networks.

A highly sensitive two-axis bending sensor based on long period fiber grating (LPFG) in a dual side hole fiber (DSHF) is presented and experimentally investigated. The LPFG is fabricated by periodically collapsing a piece of DSHF with automatic arc discharge technology. The existence of the two air holes in the DSHF makes the cladding modes, regardless of the polarization state, concentrate in the area perpendicular to the connection of the two holes. Such a feature leads to quite different bending responses at the directions perpendicular and parallel to the connection of the air holes, which makes the DSHF-based LPFG suitable for two-axis bending measurement. The bending sensitivity of the LPFG are 21.03 nm/m$^-1$ and 15.77 dB/m$^-1$ at the orthogonal directions, respectively. Besides, compared with the general arc-induced LPFGs in solid fibers, the size of the DSHF-based LPFG is effectively reduced because the periodic air hole collapse in the DSHF causes a large geometric deformation of the fiber core and increases coupling coefficient between the core and cladding modes. The sensing characteristics of strain, polarization, surrounding refractive index, and temperature are also investigated in our experiment.

Internal tensile stress inside fiber core can impact significantly on formation of Draw-tower gratings (DTGs). We carry out online dynamic preparation experiments of DTG to analyze the effect of internal stress on grating reflectivity, as well as offline multi-pulse accumulation experiments to further investigate the underlying mechanisms for grating formation. The optimum laser energy density used to grating writing and the variation of reflectivity with exposure extent were obtained, which is instrumental for their optimization and rapid prototyping. A stress modulation model was proposed as the dominant mechanism to describe the formation of the grating structure by a single laser pulse in the low-loss fiber, which is of great significance for the research of the grating photosensitivity mechanism.