A sensing system is proposed for quantitative measurement of large-range dynamic nanostrain based on a phase-sensitive optical time domain reflectometer, where the coherent detection and $I/Q$ demodulation methods are employed to demodulate both the phase and the amplitude of the Rayleigh scattering light in real time. A nanopositioning translation stage is utilized to apply precise nanostrain to fiber. By measuring phase differences between two adjacent sections, the quantitative nanostrain with a large measurement range is demonstrated; this is also a method to measure the strain parameter of refractive index. For the Panda polarization-maintaining fiber under test in the experiment, the strain parameter of phase difference is measured to be $8.714\mathrm{mrad}/(\mathrm{n\epsilon }·\mathrm{m})$, while the strain parameter of refractive index is measured to be $-0.3751{\epsilon }^{-1}$. As a proof of the concept, the dynamic strain sensing with a range of 10–1000 nε is experimentally demonstrated, and the strain resolution is 1 or 2 $\mathrm{n\epsilon }$, corresponding to 5 or 2.5 m spatial resolution, respectively. The experimental measurement also shows a triangular wave with a 12-Hz vibrating frequency and a 100-nε strain amplitude as well as a 188-Hz resonant signal of the tensile section.

We present a slope-assisted BOTDA system based on the vector stimulated Brillouin scattering (SBS) and frequency-agile technique (FAT) for the wide-strain-range dynamic measurement. A dimensionless coefficient K defined as the ratio of Brillouin phase-shift to gain is employed to demodulate the strain of the fiber, and it is immune to the power fluctuation of pump pulse and has a linear relation of the frequency detuning for the continuous pump and Stokes waves. For a 30ns-square pump pulse, the available frequency span of the K spectrum can reach up to 200MHz, which is larger than fourfold of 48MHz-linewidth of Brillouin gain spectrum. For a single-slope assisted BOTDA, dynamic strain measurement with the maximum strain of 2467.4με and the vibration frequency components of 10.44Hz and 20.94Hz is obtained. For a multi-slope-assisted BOTDA, dynamic measurement with the strain variation up to 5372.9με and the vibration frequency components of 5.58Hz and 11.14Hz is achieved by using FAT to extend the strain range.

In this paper, up to 1100°C and 1200°C high-temperature distributed Brillouin sensing based on a ${\mathrm{GeO}}_2$-doped single-mode fiber (SMF) and a pure silica photonic crystal fiber (PCF) are demonstrated, respectively. The Brillouin frequency shift’s (BFS) dependence on temperatures of the SMF and PCF agrees with a nonlinear function instead of a linear function, which is mainly due to the change of the acoustic velocity in a silica fiber. BFS hopping is observed in both kinds of fibers between 800°C–900°C in the first annealing process, and after that, the BFS exhibits stability and repeatability with a measurement accuracy as high as $\pm 2.4°\mathrm{C}$ for the SMF and $\pm 3.6°\mathrm{C}$ for the PCF. The BFS hopping is a highly temperature-dependent behavior, which means that a high temperature ($>800°\mathrm{C}$) would accelerate this process to reach a stable state. After BFS hopping, both the SMF and PCF show good repeatability for temperatures higher than 1000°C without annealing. The process of coating burning of a silica fiber not only introduces a loss induced by micro-bending, but also imposes a compressive stress on the bare fiber, which contributes to an additional BFS variation at the temperature period of the coating burning ($\sim 300°\mathrm{C}–500°\mathrm{C}$).

We propose a bend-insensitive distributed Brillouin optical fiber sensing by using a singlemode-multimode-singlemode optical fiber structure for the first time to the best of our knowledge. The sensing fiber is a graded-index multimode fiber (GI-MMF) sandwiched by two standard single-mode fibers (SMFs) with central-alignment splicing at the interface between GI-MMF and SMF to excite the fundamental mode in GI-MMF. The sensing system can resist a minimal bend radius of 1.25mm while maintain the measurement performance, with which the measured coefficients of strain and temperature are 421.6MHz/% and 0.826MHz/°C, respectively. We also demonstrate that the higher-order modes excited in GI-MMF can be easily influenced by bending, so that exciting the fundamental mode is essential for bend-insensitive distributed sensing.

A high-sensitivity distributed transverse load sensor based on Brillouin dynamic gratings (BDGs) is proposed and demonstrated experimentally for the first time, to the best of our knowledge. The principle is to measure the transverse-load-induced birefringence change through exciting and probing a BDG in an elliptical-core polarization-maintaining fiber. A distributed measurement of transverse load is demonstrated experimentally using a 10 m sensing fiber, which features high sensitivity to a transverse load with a measurement accuracy as high as $0.8\times {10}^{-3}\mathrm{N}/\mathrm{mm}$ at a 20 cm spatial resolution.

Distributed optical fiber strain sensing significantly increases the number of sensing points compared with FBG sensor, which makes it an excellent candidate for shape sensing. Theoretical analysis indicates that the spatial resolution of strain measurement is crucial to the performance of shape sensing, so a shape sensor based on differential pulse-width-pair Brillouin optical time-domain analysis (DPP-BOTDA) is proposed to improve the spatial resolution and shape sensing performance. The sensing fiber is attached on the both sides of a steel strip substrate, which enables the measurement of Brillouin frequency shifts (BFSs) of both the sides to suppress temperature crosstalk. In experiment, firstly, the dependence of BFS variation on the curvature of fiber is measured, the result of which agrees well with theory. Then the reconstruction of three shapes are demonstrated, the spatial resolution of which is 10 cm.