Force/pressure measurement has always been a focus of attention in many industrial and environmental structures, medical fields, and defense architectures. It is particularly in high demand in areas such as oil and gas wells and pipelines, geotechnical engineering, water distribution, and wastewater treatment facilities. Traditional electronic sensors are not suitable for remote monitoring, and they are sensitive to electromagnetic interference and not easily multiplexed in large-scale sensor networks. The single-point fiber optic sensor has been successfully commercialized, but in many important application areas, even dense multiplexed quasi-distributed fiber optic sensing systems cannot meet measurement requirements. Therefore, there is a strong demand for research on distributed transverse force (TF)/pressure fiber sensing. However, compared with distributed fiber sensing techniques that can measure parameters such as strain, temperature, and vibration, the basic technology for distributed TF sensing is lacking. Some indirect measurement methods using special mechanical structures to convert TF into other parameters face significant issues such as high complexity, low accuracy, and difficulty in practical application. The development of a direct distributed TF fiber sensing technology is highly desired. Previous researchers have proposed measurement techniques based on specialty fibers and single-mode fibers (SMFs) for distributed polarization properties, providing a new idea for distributed TF fiber sensing. However, due to technical limitations or performance deficiencies in the measurement systems, there have been few studies on distributed TF fiber sensing based on polarization analysis.

Based on a thorough analysis and study of previous research on distributed TF fiber sensing and potential key technologies, the authors have taken the lead in conducting research on distributed transverse pressure fiber sensing based on polarization analysis. The breakthroughs have been made in polarization-maintaining fibers (PMFs) and SMFs-based distributed TF measurement and demodulation systems, sensing medium, system performance, and typical applications. We have constructed a constructed polarization crosstalk analysis (DPXA) system without "ghost peaks", which effectively eliminates the influence of second-order crosstalk peaks on measurement accuracy (Fig. 3 and Fig. 4). We have also studied the polarization crosstalk response characteristics of PMFs and the influence of fiber coatings, demonstrating that polyimide-coated PMF is more favorable for distributed TF fiber sensing (Fig. 6 and Fig. 7). Furthermore, we have developed the equipment for PMF's axis alignment and sensing tape fabrication (Fig. 8), which enables automated 45° birefringence axis alignment for fiber sensing tape production (Fig. 9). We have verified the feasibility of distributed TF fiber sensing using the PMF and achieved high measurement resolution and repeatability (Fig. 12). We explored the feasibility of using twisted PMF and high-birefringence spun fiber (SF) as TF sensing media without dependence on the force-applying angle. By utilizing the SF, force-applying-angle-insensitive distributed polarization crosstalk measurement within 2 dB was achieved (Fig. 16). We have invented and built a high-performance distributed polarization analysis (DPA) system with full Mueller matrix measurement capability (Fig. 18). This system enables distributed birefringence measurement in a SMF with high spatial and measurement resolution (Fig. 19 and Fig. 20). We were the first to achieve direct distributed TF sensing in a SMF (Fig. 21) and obtained excellent sensing performance (Table 1, Fig. 24, and Fig. 25). We validated the feasibility of TF measurement-based monitoring deformations in SMF-embedded composite materials (Fig. 28) and determined the groove-angles for zero clamping-induced birefringence when fixing the SMF in two types of V-grooves (Fig. 30).

The above research findings provide a foundation for the measurement and demodulation techniques of distributed TF fiber sensing and provide a good overall technical reserve for its continuous promotion to practical applications. In the future, the development will mainly focus on miniaturization and integration of sensing demodulation systems, low-cost mass production of PMF and SF, optimization of fiber coating processes, or development of new coating materials to enhance the sensitivity of TF sensing, practical online applications of distributed TF fiber sensing, and exploration of new directions. Additionally, the DPA technology has also shown a potential advantage in monitoring the deformation of SMF-embedded composite materials and characterizing and optimizing birefringence properties within optical fiber devices and optical equipment. It is also a key direction for future development.