Spherical LPFG high-sensitivity strain sensor based on V-shaped fiber core

Compared with the weak refractive index modulation type long-period fiber grating (LPFG) , the strong refractive index modulation type LPFG proposed in recent years can effectively reduce the length of the sensing area, but has the disadvantage of low sensitivity. This article proposes a spherical strong refractive index modulation LPFG (SP-LPFG) based on a V-shaped fiber core to achieve high-sensitivity strain sensing. CO₂ laser is used to etch V-grooves on single-mode fiber, and fiber balls are fabricated at the V-grooves using a fusion splicer to form a spherical structure with V-shaped fiber core. The light inside the fiber core will directly enter the cladding in the middle of the V-shaped fiber core for transmission, achieving strong refractive index modulation. Because of the large diameter of the fiber spherical region, the strain is concentrated in the straight fiber region when subjected to strain, resulting in a much larger length change in the non-refractive index modulation region than in the refractive index modulation region. This changes the duty cycle of the strong refractive index modulation region, changes the effective refractive index modulation depth, introduces additional resonant wavelength shift, and achieves strain sensitivity enhancement. The findings of the experiment demonstrate that the SP-LPFG features a compact sensing area with a length of merely 2.88 mm. Additionally, it exhibits an impressive strain sensitivity of 57.1 pm/µε. Moreover, its temperature cross sensitivity is remarkably low, registering at just 0.702 µε/℃. The proposed SP-LPFG provides a novel approach to improving the sensitivity of strong refractive index modulated LPFG strain sensor, which can be integrated into precision structure for strain detection.